Cannot access property on Swift type from Objective-C

Question

I am trying to access a Swift class's Double? property from Objective-C.

class BusinessDetailViewController: UIViewController {      var lat : Double?     var lon : Double?      // Other elements... } 

In another view controller, I am trying to access lat like following:

#import "i5km-Swift.h" @interface ViewController ()  @property (strong, nonatomic) BusinessDetailViewController *businessDetailViewController;  @end  @implementation ViewController  - (void)viewDidLoad {     [super viewDidLoad];     self.businessDetailViewController = [[BusinessDetailViewController alloc] initWithNibName:@"BusinessDetailViewController" bundle:nil];     self.businessDetailViewController.lat = businessArray[1]; /* THIS GIVES ME AN ERROR */ } 

and I am getting

Property 'lat' not found on object of type 'BusinessDetailViewController *'

Why can't I access this property? What am I missing?

Answer 1

Optional values of non-Objective-C types aren't bridged into Objective-C. That is, the first three properties of TestClass below would be available in Objective-C, but the fourth wouldn't:

class TestClass: NSObject {     var nsNumberVar: NSNumber = 0      // obj-c type, ok     var nsNumberOpt: NSNumber?         // optional obj-c type, ok     var doubleVar: Double = 0          // bridged Swift-native type, ok     var doubleOpt: Double?             // not bridged, inaccessible } 

In your Objective-C code, you'd access those first three properties like this:

TestClass *optTest = [[TestClass alloc] init]; optTest.nsNumberOpt = @1.0; optTest.nsNumberVar = @2.0; optTest.doubleVar = 3.0; 

In your case, you can either convert lat and long to be non-Optional or switch them to be instances of NSNumber.

---

Note that you need to be careful about your Objective-C code if you take the second approach (switching lat and lon to non-optional properties of type NSNumber) -- while the Swift compiler will prevent you from assigning nil to non-optional properties, the Objective-C compiler has no qualms about allowing it, letting nil values sneak into your Swift code with no chance of catching them at runtime. Consider this method on TestClass:

extension TestClass {     func badIdea() {         // print the string value if it exists, or 'nil' otherwise         println(nsNumberOpt?.stringValue ?? "nil")          // non-optional: must have a value, right?         println(nsNumberVar.stringValue)     } } 

This works fine if invoked with values in both of the properties, but if nsNumberVar is set to nil from the Objective-C code, this will crash at runtime. Note that there is no way to check whether or not nsNumberVar is nil before using it!

TestClass *optTest = [[TestClass alloc] init]; optTest.nsNumberOpt = @1.0; optTest.nsNumberVar = @2.0; [optTest badIdea]; // prints 1, 2  optTest.nsNumberOpt = nil; optTest.nsNumberVar = nil; [optTest badIdea]; // prints nil, then crashes with an EXC_BAD_ACCESS exception